Powder mixtures having a cellulose ether binder are used in forming articles of various shapes. For example ceramic powder mixtures are formed into honeycombs which are used as substrates in catalytic and adsorption applications. The mixtures must be well blended and homogeneous in order for the resulting body to have good integrity in size and shape and uniform physical properties. The mixtures have organic additives in addition to the binders. These additives can be surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
A major and ongoing need in extrusion of bodies from highly filled powder mixtures, especially multicellular bodies such as honeycombs is to extrude a stiffer body without causing proportional increase in pressures. The need is becoming increasingly critical as thinner walled higher cell density cellular structures are becoming more in demand for various applications. Thin walled products with current technology are extremely difficult to handle without causing shape distortion.
Rapid-setting characteristics are important for honeycomb substrates. If the cell walls of the honeycomb can be solidified quickly after forming, the dimension of the greenware will not be altered in subsequent cutting and handling steps. This is especially true for a fragile thin-walled or complex shaped product, or a product having a large frontal area.
More recently, attempts to extrude stiffer ceramic batches with the current batch components, i.e. cellulose ether binder, involving use of various organic materials in the forming mixture have been successful. One drawback of using organic materials, however, is that they have to be removed from the shaped green structure during the firing cycle. This generally results in cracking of the body possibly due to pressure build up inside the structure and/or differential heat produced by combustion of the organics in the early stages of firing.
The growing need for thinner webs (1-2 mil)/high density cellular products to be extruded to shape necessitates stiffer batches; and certain organics in the batch contribute to stiffening of the green structures. Therefore, a method to avoid the cracking during organic removal is highly desirable and would be a significant advancement in the art.
The present invention provides such a method.